Piping systems and pipelines are subject to defects such as, arc burns, corrosion, cracks, dents, fretting, gouges, and grooves that compromise structural integrity. (A person of ordinary skill in the art would recognize that a composite load transferring technique according to this invention is equally applicable to piping as it is to pipeline. Rather than use terms such as “pipeline/piping” or “pipeline/piping system” throughout this disclosure, pipeline is used instead.) Because of the potential of a defect to cause catastrophic failure, pipeline operators employ various external and internal inspection methods to evaluate pipeline conditions and identify defects. When a defect is identified, various repair methods are employed based upon such factors as defect location, type, and size. Repair methods include grinding, weld deposit, sleeves, clamps, and hot tapping. Preferably, operators would like to make the repair without having to shutdown or reduce the flow of the pipeline.
An advance in sleeve-type repairs has been the use of composite materials. The composite is typically multiple layers of carbon, glass, or aramid fibers bound together by a polymeric matrix consisting of either epoxy, polyurethane, or vinlyester in the form of a patch or wrap. First, the surrounding pipeline surfaces are prepared to receive the composite wrap and filler by grit-blasting or an equivalent process. In a typical repair, putty filler is used to fill any voids in the pipeline created by the defect and to taper uneven welds or misaligned pipes. The surface is then prepared with low viscosity polymeric primer to ensure bonding and load transfer between the repair and the substrate. The structural reinforcing fibers, or fabric, are then saturated with a liquid polymer and the wet fibers are wrapped around the outer pipeline surface. The wrap is then allowed to cure at ambient temperature and atmospheric pressure.
Composite wrap repairs can be difficult and labor intensive due in part to the handling of the wet fibers and the time-sensitive nature of the liquid polymer. As the polymer set-up time or pot life expires, the liquid polymer becomes more viscous and difficult to mold and shape. Unlike cure time, which may be a day or several days, the pot life of many liquid polymers is only a few minutes. One method for addressing the pot life problem is to apply dry fabric to the pipeline surface and then optionally enclose the fabric with a vented clamp, sleeve or shell into which liquid polymer is injected. (See U.S. Pat. No. 7,387,138, issued to Rice et al., Jun. 17, 2008) (hereinafter, “the '138 patent”).
Other types of composite wrap systems include a pre-impregnated system and pre-cured coil. A pre-impregnated system is one that has a polymer applied onto the fibers at the factory; however, the polymer is not fully cured at this stage. Reaction of the polymer is achieved by the addition of heat or a chemical (including water) to the pre-impregnated fiber. This means that a liquid polymer is applied to dry fibers at a factory and the reaction is suspended until heat or some type of chemical is added to the system once it is applied to the pipeline.
In a pre-cured coil, the repair system is shipped from the factory with the polymer completely reacted onto the fibers. Each layer of the repair system is therefore pre-cured and is pre-formed to the pipeline outer diameter. In the field, this pre-cured coil is pulled around the pipeline and an adhesive is applied to each layer to bond the coil together.
The condition of current composite systems is such that pipeline stresses are only shared above the internal pipeline pressure at which the wrap was applied. It is not, however, practical to lower pressure near or to ambient conditions during curing to obtain maximum load transfer. When pipeline pressure is increased, the wrap will begin to share the load with the pipeline once the pipeline wall expands to the diameter at which the composite wrap was installed. For example, the '138 patent does not equalize the pressure in the shell's cavity with that of the pipeline pressure when injecting the liquid polymer into the cavity. Rather, the pressure applied is the pressure effective for obtaining polymer impregnation of the dry fabric and displacing any entrapped air in the cavity. The shell is “sealed” only to the extent needed to contain the liquid polymer and allow it to effectively impregnate the fabric. Therefore, a need exists to provide conditions under which the composite cures while the pipeline is operating but reduce the diameter of the pipeline as if the pipeline pressure had been reduced below operating pressure.